In fabricating microelectronic semiconductor devices and the like on a wafer substrate or chip, e.g., of silicon, to form an integrated circuit (IC), etc., various metal layers and insulation layers are deposited in selective sequence. To maximize integration of device components in the available substrate area to fit more components in the same area, increased IC miniaturization is utilized. Reduced pitch dimensions are needed for denser packing of components per present day very large scale integration (VLSI), e.g., at sub-micron (below 1 micron, i.e., 1,000 nanometer or 10,000 angstrom) dimensions.
One type of wet chemical process used in the IC fabrication of a semiconductor wafer concerns the cleaning of the wafer to remove contaminant particles from its surface. This may be effected by immersing the wafer in a hot deionized water cleaning bath subjected to rapid agitation such as by applying non-reactive cleaning enhancing (bubble generating) gas, e.g., nitrogen (N.sub.2), and/or megasonic vibrations thereto.
For overall cleaning of the wafer, e.g., of silicon, a so-called "RCA clean" process has been used wherein the wafer is treated with two cleaning agents in sequence comprising an alkaline, so-called SC1 (standard clean 1), solution, e.g., of hydrogen peroxide (H.sub.2 O.sub.2) and ammonium hydroxide (NH.sub.4 OH) in deionized water, such as for removing organic and particulate contaminants, in a first step, and then an acidic, so-called SC2 (standard clean 2), solution, e.g., of hydrogen peroxide and hydrogen chloride (HCl) in deionized water, such as for removing metallic impurities, in a second step. Each treatment step is effected, e.g., for about 10-20 minutes at about 75-85 .degree. C., and is followed by a rinse step typically using hot deionized water. The wafer is usually dried in a drying step after the final rinse step.
For removing particles in particular, a traditional SC1 mixture of deionized H.sub.2 O/H.sub.2 O.sub.2 /NH.sub.4 OH at a volume ratio of about 5:1:1 has been used to clean the wafer such as at about 65.degree. C. for about 10 minutes. The high concentrations of the SC1 chemicals in the solution and high temperature used cause removal of most particles by etching the wafer surface and the particles to some extent, thus reducing the particle adhesion forces with the wafer and promoting particle movement away from the wafer and into the bulk of the solution. The high pH of the SC1 solution also induces negative charges on the wafer and particles, providing a mutual repulsion tending to keep loosened particles from reattaching to the wafer surface. However, such traditional SC1 cleaning solution is expensive and too aggressive at many critical cleaning steps for use in currently available devices.
Recent introduction of megasonics assisting techniques into wafer cleaning processes has led to better particle removal efficiency with solutions substantially less aggressive and thus less harmful to the surface of the wafer, e.g., of silicon. The megasonic vibration assisting cleaning solutions are usually dilute versions of the traditional SC1 solution, and are used at widely varying temperatures depending on the effect sought. A typical dilute SC1 solution used in this regard is a mixture of deionized H.sub.2 O/H.sub.2 O.sub.2 /NH.sub.4 OH at a volume ratio of about 100:0.9:0.5, wherein 98+% (100/101.4=98.6) is deionized water and only about 1.4% constitutes the active chemicals. Because almost all of the solution is water, the amount of gases dissolved therein will dominate the total gas concentration of the dilute SC1 mixture.
The exact mechanism by which megasonics assisting techniques enhance the particle removal operation is not fully understood at this time. However, as noted below, it is clear that the amount of dissolved gases in the cleaning solution is critical for effective cleaning, i.e., particle removal, to occur.
Some examples of methods of cleaning semiconductor wafers are shown in the following prior art.
U.S. Pat. No. 5,464,480 (Matthews), issued Nov. 7, 1995, discloses removing organic material, e.g., photoresist, from a semiconductor wafer in a tank with sub-ambient or chilled (1-15.degree. C.) deionized water in which ozone (O.sub.3) is diffused, and then rinsing the wafer with deionized water. While ozone has little solubility in deionized water at room temperature or higher temperature, it is stated to be sufficiently soluble therein at sub-ambient temperature to oxidize the organic material to insoluble gases. Megasonic transducers are used to agitate the ozonated deionized water in the tank.
For an RCA clean therein, the wafer is rinsed with deionized water, treated with ozonated deionized water in which ammonia (NH.sub.3) is diffused to form an SC1 solution, and rinsed again. Next, hot (70.degree. C.) deionized water is used to raise the tank temperature. Then, the wafer is treated with deionized water in which ozone gas and hydrochloric (i.e., hydrogen chloride, HCl) gas are diffused to form an SC2 solution, whereupon the wafer is finally rinsed with deionized water.
U.S. Pat. No. 5,714,203 (Schellenberger et al.), issued Feb. 3, 1998, discloses dipping a silicon wafer in an aqueous cleaning bath containing hydrogen fluoride (HF), which renders the wafer surface hydrophobic, and removing the wafer therefrom while subjecting the bath surface, or alternatively the removed and dried wafer, to a gaseous flow of an oxygen/ozone (O.sub.2 /O.sub.3) gas mixture alone or in a carrier gas chemically inactive thereto, such as air, i.e., nitrogen, oxygen and carbon dioxide (N.sub.2, O.sub.2 and CO.sub.2), or carbon dioxide, helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) or radon (Rn). When applied to the bath surface, the gaseous flow lowers the liquid surface tension to aid drying of the wafer, and when applied to the dried wafer, the gaseous flow hydrophilizes the wafer surface. The wafer surface is also hydrophilized if the cleaning bath contains ozone.
U.S. Pat. No. 5,569,330 (Schild et al.), issued Oct. 29, 1996, discloses chemically treating a semiconductor wafer in sequence in the same container with a liquid bath containing HF which renders the wafer surface hydrophobic, then with a liquid bath containing ozone which renders the wafer surface hydrophilic, while applying megasonic vibrations thereto in both treating steps, and finally drying the wafer. The wafer may also be intermediately dried between the treating steps.
U.S. Pat. No. 5,520,744 (Fujikawa et al.), issued May 28, 1996, discloses treating a silicon wafer in a hermetically sealed chamber in sequence with three constant temperature heated (e.g., 60.degree. C.) chemical baths of deionized water, respectively containing (1) hydrogen peroxide and ammonia, (2) HF, and (3) hydrogen peroxide and HCl, and also with a deionized water rinsing bath after each chemical bath treatment. A vapor of an inactive gas and an organic solvent, e.g., nitrogen and isopropyl alcohol, is applied to the wafer after the last rinsing bath to lower the surface tension of the deionized water remaining thereon to aid drying of the wafer, which is effected under reduced pressure, while reducing adhesion of contaminant particles thereto.
U.S. Pat. No. 5,800,626 (Cohen et al.), issued Sept. 1, 1998 (having a common inventor herewith, and assigned to International Business Machines Corporation), discloses a method for controlling the effectiveness of megasonics assisted cleaning of a substrate of a microelectronics device with a cleaning solution containing deionized water and gas at a given process temperature. This is done by vacuum degassing the water and then adding gas, e.g., nitrogen, back thereto in an amount to provide a cleaning solution only partially, e.g., 60-98%, saturated with the gas at the given process temperature. The cleaning solution is a dilute solution of deionized H.sub.2 O/H.sub.2 O.sub.2 /NH.sub.4 OH, e.g., in a volume ratio of 10:1:1 to 1,000:2:1, respectively (SC1), or of deionized H.sub.2 O/H.sub.2 O.sub.2 /HCl, e.g., in a volume ratio of 10:0:1 to 1,000:1:1, respectively (SC2). A first portion of vacuum degassed deionized water and a second portion of at least partially gas saturated deionized water can be mixed in a ratio effective to provide the only partially gas saturated water used at the given process temperature.
Said U.S. Pat. No. 5,800,626 notes that a higher temperature or a lower applied pressure can reduce the amount of gas that can be dissolved in the solution, such that heating gas saturated water causes some of the dissolved gas to be expelled via bubbles. One danger is that having too much gas in a wafer cleaning solution can lead to formation of gas bubbles in hot deionized water per temperature driven saturation, causing defects in silicon surfaces. Also, the presence of oxygen in particular in deionized water can cause etching and roughening of hydrogen-terminated silicon surfaces since oxygen gas is considered reactive thereto although inert to oxide wafer surfaces. For these reasons, deionized water is typically provided in degassified form, and the degassified water regassified with a particular gas of specific solubility and temperature dependency to provide an only partially gas saturated water for use at the given process temperature. While the provision for only partially gas saturated water enables the substrate cleaning to be effected at lower megasonic power, lower temperature and much lower concentrations of chemicals, such preparation of only partially gas saturated water is limited to use at only one given process temperature.
The disclosure of said U.S. Pat. No. 5,800,626 is incorporated herein by reference.
It is clear that the gas concentration, i.e., of a non-reactive cleaning enhancing (bubble generating and agitating) gas, in deionized water used for megasonic cleaning of semiconductor wafers, e.g., of silicon, has a strong influence on particle counts, i.e., the amount of contaminating particles remaining after cleaning, compared to the original amount thereof present before cleaning.
In this regard, for hydrophilic wafers, e.g., using standard cleans SC1 and SC2, the usual application of megasonic vibrations requires the cleaning bath to have a high concentration of cleaning enhancing, i.e., agitation imparting, gas therein. On the other hand, for hydrophobic wafers, e.g., using HF processing, a high gas concentration in the cleaning bath, with consequent excessive formation of bubbles, is detrimental and usually results in high particle counts, i.e., high amounts of contaminating particles remaining on the wafer after cleaning. This is because gas bubbles tend to nucleate at, or migrate to, hydrophobic surfaces and deposit particles thereat. Hence, for hydrophobic wafers, a dilute, e.g., HF, solution is used having a dissolved gas content well below its saturation concentration.
Because dilute cleaning solutions are predominantly deionized water, particular attention must be paid to the amount of dissolved gas in the deionized water used in present day wet cleaning tools for cleaning semiconductor wafers.
In order to allow an optimum gas concentration in the liquid bath used for each treatment step in a given cleaning operation, e.g., of sequential washing and rinsing steps, present day wet cleaning tools used for such purposes are in some cases equipped with a gas adjustment component in the form of a gassifier/degassifier, such as a so-called contactor. The component is typically a sealed chamber divided by a gas permeable membrane into a liquid compartment (water space) and a gas compartment (gas space), with water being supplied to the water space.
When the component is used as a gassifier, gas is supplied to the gas space by a pump applied as a pressure pump and in turn via the membrane to the water at a selective positive pressure to adjust the concentration of the gas dissolved in the water by increasing such concentration. On the other hand, when the component is used as a degassifier, gas is removed from the water via the membrane and in turn from the gas space by the pump applied as a suction pump at a selective vacuum pressure to adjust the concentration of the gas dissolved in the water by decreasing such concentration.
The gassifier is usually positioned on the tool before, i.e., upstream of, the heater used to heat the, e.g., room temperature (cold) deionized water to the (hot) cleaning temperature, upon transfer thereto from the gassifier, prior to transfer of the water to the cleaning tank for semiconductor wafer cleaning. However, where both room temperature (cold) and preheated (very hot) deionized water supplies are available, the cold and very hot supplies can be mixed in predetermined proportions to provide water at the hot cleaning temperature.
However, none of the present day wet cleaning tools take into account that at higher temperatures, an over-saturation (super-saturation) of the gas in the deionized water of the cleaning bath can take place. This reduces significantly the megasonic vibration efficiency during the cleaning step.
It is desirable to have a system, including a method and an apparatus arrangement, permitting selective adjustment of the gas concentration in the deionized water before it is heated to a selective elevated cleaning temperature for cleaning semiconductor wafers under megasonic vibration action, so as to avoid inefficient over-saturation or under-saturation of the gas in the thusly heated deionized water.